Various methods and apparatus have been developed to perform fine-resolution imaging in the presence of time-varying aberrations encountered, for example, when imaging through a turbulent medium (such as the atmosphere) or when the optical system is mechanically unstable. Known approaches include stellar speckle imaging (shown in FIG. 2), which requires the collection of many short-exposure images (specklegrams) of the same object. Each short-exposure image is formed through a different atmospheric realization. The exposure time for each of these multiple atmospheric realizations must be short enough (about 10 milliseconds) that the evolving atmosphere can be regarded as frozen during exposure. Current stellar speckle imaging approaches perform an averaging of the data as part of the process of individually estimating the Fourier modulus or the Fourier phase of the object. The final estimated object is then constructed by combining the Fourier modulus and phase estimates and performing an inverse Fourier transform. Speckle imaging also requires a cumbersome atmospheric-calibration step utilizing images of an unresolved object seen through atmospheric turbulence having the same statistics.
Reconstruction of the object from a sequence of specklegrams could be substantially improved if estimates for the individual point spread functions (PSFs) for each specklegram were available. This is the rationale behind deconvolution with wavefront sensing (also referred to as self-referenced speckle holography), which employs a Hartmann-Shack wavefront-sensor to estimate the PSFs for each atmospheric realization. However, this wavefront-sensor is a relatively complex optical system that must be carefully aligned. Moreover, the performance of the wavefront-sensor can degrade with object extent.
In compensated imaging, an attempt is made to sense the aberrated wavefront and correct it with a deformable mirror prior to detection. When it can be successfully accomplished, pre-detection correction is preferable to post-detection correction since it typically operates with a better signal-to-noise ratio. There are a variety of reasons, however, that compensated imaging may not be a completely satisfactory solution for imaging through turbulence in every case. Compensated imaging systems are typically very expensive, complicated, and sensitive to systematic errors. Maintenance and calibration are ongoing tasks and the risk of system failure cannot be overlooked. Also, the compensation will never be perfect and there may be a need for post-detection correction of residual errors.
Some researchers have proposed adaptive correction of anisoplanatic (space-variant) effects through the use of multiple guide stars, multiple wavefront sensors, and/or multiple deformable mirrors in a single system. Whereas these proposed schemes are intriguing, adaptive systems utilizing combinations of these techniques would likely be extremely complex.
U.S. Pat. No. 4,309,602, issued to Gonsalves et al., for "Wavefront-Sensing by Phase Retrieval" discloses a method of phase diversity which may be regarded as an indirect wavefront-sensor since phase aberrations are estimated from image data. Phase diversity requires the collection of two or more phase-diverse images, one of which is the conventional focalplane image that has been degraded by the unknown aberrations. Additional images of the same object are formed by perturbing these unknown aberrations in some known fashion. This can be accomplished with very simple optical hardware by, for example, utilizing a simple beam splitter and a second detector array translated along the optical axis, which further degrades the imagery with a known amount of defocus. The goal is to identify a combination of object and aberrations that is consistent with all of the collected images, given the known phase diversities. Phase diversity (shown in FIG. 3) was disclosed as a component in a compensated imaging system. However, phase diversity could also be used to create a post-detection estimate of the object. In the case of faint objects, however, the signal may not be strong enough to get a high fidelity object estimate with only a single aberration realization.
One object of the present invention is, therefore, to provide a method and apparatus for recovering a fine-resolution image of an object and identifying aberrations including a data collection and processing approach for imaging in the presence of phase aberrations, such as atmospheric turbulence, in which only simple optical hardware is required.
Another object of the present invention is to provide a method and apparatus for acquiring and estimating images which is robust to systematic error.
Another object of the present invention is to provide an apparatus for acquiring and estimating images which requires little or no calibration.
Another object of the present invention is to provide a method and apparatus for estimating images in which there is no averaging of the data nor use of intermediate estimates in which information is lost.
Another object of the present invention is to provide a method and apparatus for improving the quality of object estimates over estimates that can be achieved from a single aberration realization.